When this mod is loaded and post-initialized I see extremely high number of threads opened in minecraft process and most, if not all, do not do any work.
These threads persist until minecraft process closed so I suppose these threads are created and left floafting around in the process without any use.
I don't think this can be caused by Optifine, so please don't reject this issue only because of its existence

Screenshot taken from utility program called Process Hacker:

System configuration / logs:
Minecraft version: 1.12.2
JEI Version: 4.16.1.302
JER Version: 0.9.2.60
latest.log: https://gist.github.com/MadProbe/f8e93c161be3d74738746c41729217f6

There is quite a large number of other mods install, what makes you suspect this is JER? Can you reproduce with only JER installed?

Just thinking out loud, but if I recall correct thaumcraft creates a bunch of threads during init.

I have opened this issue only because when I add your mod into my modpack I can reproduce this issue.
Also I see this issue when stage 3 of minecraft launch is ended and until minecraft is closed and not just during world init!
Regarding Thaumcraft: It creates threads for aura calculations in chunks and their number is equal or less than number of dimensions at world initialization and it closes threads if dimension inactive (unloaded).
But I understand that you request me to check JER only installation and I will write results of my research after some time.

Personally I would mark this issue as mod compat since I have found that somehow your mod and mod named perfomant are causing the issue when they are moth active.
The mods alone don't cause issues but together they somehow conflict and in the result I get a lot of inactive threads which amount depends roughly on count of mods and grows expondentially as count of mods grows.

Closing since I don't support 1.12 anymore